I'm looking for a way to know whether certain lights are on. These are ones that can not be controlled by Lutron, for various reasons (old buildings, outdoor lights on dusk/dawn sensors). The ideal (and maybe only) solution that I can think of is a light bulb socket coupler that transmits when it has current. In a scenario like that, I could work on how to make it talk to my Crestron system.

Is anyone familiar with anything like this...or another retrofit-style to detect whether a specific bulb is on?

Any other current sensor or light sensor (if it wouldn't be affected by sunlight) could work too. Seems like you should be able to integrate your outdoor lights with either a timeclock or additional light sensor (outdoor).

Thanks az1324. My goal is the other way around; less about when I turn them on, and more about knowing when they turn on.

Another use case I'm tinkering with is knowing when the garage door opener light turns on. This indicates when the garage doors are opened and also when someone switches them on manually. Detecting current at the bulb would help me capture that event for logging and other automation purposes.

Well if you turn them on then you know when they turn on. Plus then you can override them at will. It seems much more useful to have a daylight sensor as an input to your system and use it to trigger groups of lights or other things as you want.

How do you know the difference between when the garage opens and when someone just turns the light on? Not to mention open vs close. Why not just use an open/close sensor?

Do you really have to do it by the bulb? How many bulbs do you need to monitor? Can't you do it by the circuit and just use one sensor for each circuit?
For your garage lights (inside the door opener) install a cadnium sulfied cell and run it through a versiport. That will show open line in the total absence of light up to about 30 ohms from a regular light bulb. To much work for my taste.

Current owner of the last/best AmPro on the planet. The mighty 4600HD, and it's still running...better than Barco's, especially southern ones.

I had a similar application - the wall switch next to my front door only controlled an outlet, so I gutted an X10 appliance module, replaced the guts with a 110V coil voltage relay, and used the contact closure outs on the relay to tell my AMX system to turn on the overhead light.

I do know of a few commercially available product that might help. Most of these are designed to detect device power - sensing the "on" state of devices with toggling power commands. So they are all plug based.

mbarland's INSTEON solution is handy since you wouldn't need to run additional wires, but that might take additional hardware. I also know X10 has a device that senses a contact closure or 12V input and sends an X10 pulse.

A long time ago, Crestron used to make a light sensor that could be glued over the power light of a device and, if the power light was on, sent a trigger to the Crestron. Slick little device, but sadly, they didn't make them for long. Crestron CNLEDS.